Thinking Of You,
by Auric Goldfinger.
One beautiful, sunny morning in May, in a little town in the United States of America, a funeral is going on. A 16 year old girl mourns the death of her mother. Between the crowd, there is a boy strolling around and despite the grief for her mother, the girl falls instantly in love with him. She manages to obtain his phone number and they agree to call a week later, after the mourning ceremonies, in order to meet each other again.
Once at home however, the girl discovers she has lost the phone number of the boy.
A week after her mother was buried, the girl murders her four years older sister.
What was her motive?